Determining the distance to measurement points forms the basis for a multiplicity of measurement tasks and for corresponding measuring apparatuses. Optical distance measurement, in particular, is used for example for measuring apparatuses in surveying (geodesy) or in industrial workpiece checking or measurement. This can involve, for example, determining coordinates of points in the terrain or on a workpiece to be monitored. Advantages of these methodologies include, in particular, a broad field of application on account of the large measurement range and the comparatively high measurement accuracies which can be provided e.g. by interferometer distance measurement.
For measuring a target point, numerous geodetic surveying apparatuses have been known since ancient times. In this case, direction or angle and usually also distance from a measuring apparatus to the target point to be measured are recorded and, in particular, the absolute position of the measuring apparatus together with reference points possibly present are detected as spatial standard data.
Generally known examples of such geodetic surveying apparatuses include the theodolite, tachymeter, total station and also laser scanner, which are embodied in the terrestrial and airborne variants. One geodetic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such apparatuses have electrical-sensor-based angle and distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the apparatus and, if appropriate, also have to be combined with an external reference system for absolute position determination.
Modern total stations have microprocessors for digital further processing and storage of detected measurement data. The apparatuses generally have a compact and integrated design, wherein coaxial distance measuring elements and also computer, control and storage units are usually present in an apparatus. Depending on the application, total stations are additionally equipped with motorization of the targeting or sighting device and—in the case of the use of retroreflectors (for instance an all-round prism) as target objects—means for automatic target seeking and tracking. As a human-machine interface, the total station can have an electronic display control unit—generally a microprocessor computing unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data detected in an electrical-sensor-based manner are fed to the display control unit, such that the position of the target point can be determined, optically displayed and stored by the display control unit. The use of optical distance measurement makes it possible in this case for distances to be determined in a precise manner over large distances to measurement point comparatively far away.
Moreover, in many technical or industrial areas of application there is a need to measure surfaces of objects and thus also the objects themselves with high accuracy. This applies in particular to the manufacturing industry, for which the measurement and checking of surfaces of workpieces are of great importance, in particular also for quality control purposes.
Coordinate measuring machines are usually used for these applications, said coordinate measuring machines enabling precise measurement of the geometry of an object surface, typically with micrometer accuracy. Objects to be measured may be, for example, engine blocks, transmissions and tools. Known coordinate measuring machines measure the surface by producing a mechanical contact and scanning the surface. Examples thereof are gantry measuring machines, as described e.g. in DE 43 25 337 or DE 43 25 347. A different system is based on the use of an articulated arm, whose measuring sensor arranged at the end of the multipartite arm can be moved along the surface. Generic articulated arms are described for example in U.S. Pat. No. 5,402,582 or EP 1 474 650.
Moreover, in the meantime it has become customary to use optical measuring sensors in coordinate measuring machines. The optical sensors used for this purpose are based on e.g. laser light being radiated onto an object surface for interferometric measurements (EP 2 037 214). Methods based on white light interferometry (DE 10 2005 061 464) and chromatic-confocal methods (FR 273 8343) are also known.
Optical sensors and measuring methods for a coordinate measuring machine are associated with a series of advantages: the measurement is carried out contactlessly, and the optical sensor can be led over an object surface more rapidly than a tactile sensor, with a smaller physical dimensioning of the “measuring tip”, as a result of which a higher lateral resolution of the measurement is made possible.
However, the optical measuring methods mentioned share the disadvantage of distance determination of only limited accuracy when unfavorable environmental influences occur, such as e.g. vibrations on the measuring apparatus, or on surfaces that are difficult to measure, which e.g. cause great scattering of the measurement radiation or have a roughness that is unfavorable with regard to the radiation properties chosen. In this case, so-called “speckle effects” can occur, which can contribute significantly to measurement uncertainty, and as a result individual recorded measurement values with respect to a defined point can be subject to great fluctuations.
More specifically, geometrical shape detection by means of optical sensors (e.g. with coordinate measuring machines) in the course of individual-point measurements on surfaces having a roughness in the range of the optical wavelength of the measurement radiation suffers from measurement uncertainties on account of the coherence of the emitted or detected radiation. In the case of narrowband light sources such as lasers, for example, the coherence of the radiation is predefined by the line width and in interferometric measurements is manifested during detection in so-called “speckles”, which leads to a modulation of the detected amplitude and phase.
Although certain approaches, such as e.g. taking account of amplitude weighting in the phase determination, allow the speckle influence to be reduced, they do not allow complete elimination thereof (cf. B. Wiesner et al., “Improved white-light interferometry on rough surfaces by statistically independent speckle patterns”, Appl. Opt. 51, 751-757 (2012) and EP 2 037 214 A1). In such methods, the remaining uncertainty is in the range of the roughness of the measurement object (see Paval Pavliček and Jan Soubusta, “Theoretical Measurement Uncertainty of White-Light Interferometry on Rough Surfaces”, Appl. Opt. 42, 1809-1813 (2003)).
Non-phase-evaluating methods such as chromatic-confocal metrology also exhibit uncertainties on account of speckles, which arise in this case not as a result of the coherence of the source but rather as a result of the spectral filtering in the context of the measuring method. As a result, only a narrow range of the measurement radiation used is effectively available, which corresponds to a reduction of the line width of the source and can thus be equated with an increase in coherence (D. Fleischle, W. Lyda, F. Mauch, and W. Osten, “Untersuchung zum Zusammenhang von spektraler Abtastung and erreichbarer Messunsicherheit bei der chromatisch-konfokalen Mikroskopie an rauen Objekten” [“Investigation of the relationship between spectral scanning and achievable measurement uncertainty during chromatic-confocal microscopy on rough objects”], DGAO Proceedings 2010).
With regard to the measurement uncertainty caused by speckles, during spectrally resolved white light interferometry—also called Fourier-domain Optical-Coherence-Tomography (FD-OCT)—the measurement radiation is spectrally decomposed in a spectrometer with corresponding speckle sensitivity (D. C. Adler, T. H. Ko, and J. G. Fujimoto, “Speckle reduction in optical coherence tomography images by use of a spatially adaptive wavelet filter”, Opt. Lett. 29, 2878-2880 (2004)).
The accuracy of scanning white light interferometry with a variable, tunable reference arm length—also called time-domain OCT—is likewise speckle-dependent. In this case, the emission width can be regarded as a bundle of individual, narrowband wave packets which are brought to interference in the case of equidistant arm lengths. The roughness of the measurement object then leads to an amplitude and phase modulation of the interferogram (A. Harasaki, J. C. Wyant, “Fringe modulation skewing effect in white-light vertical scanning interferometry”, Appl. Opt. 39, 2101 (2000)).
What is common to these methods is the statistical fluctuation of the distance value during measurement point recording on rough surfaces. As a result, shape and dimensional errors of a few micrometers can occur during individual measurements e.g. on a slightly roughened surface of a calibration sphere.
Therefore, it is an object of the present invention to provide an improved method for measuring distance and an improved measuring apparatus, whereby a measured distance to a point can be determined more precisely and more reliably, in particular with a lower measurement uncertainty.